Polymer based positive temperature coefficient resistors (PTC resistors) are well known in the art, and are finding increasing use in a great variety of applications for protecting low voltage electrical circuits from the flow of excessive currents, much like a fuse or a circuit breaker. Some of the polymer based PTC resistors are sold commercially under the registered trademark "PolySwitch" by Raychem Corporation, 300 Constitution Drive, Menlo Park, Calif., and utilize one of several conductive polymer compounds which have an exceptionally low level of resistance to electrical flow at ambient or normal operating temperatures, and in which the electrical resistance increases quite abruptly and dramatically upon an increases in temperature. As utilized to protect an electrical circuit, the circuit's electrical current is directed through the PTC resister at normal current levels, which will readily flow therethrough, providing exceptionally little electrical resistance to the flow as long as the temperature remains below a given value, which is an inherent characteristic of the particular polymer used, and is appropriately selected for the particular application of use. When the current increases above a predetermined, normal operating current level, as a result of voltage surge, a fault or any other cause, the temperature of the PTC resistor, i.e., the conductive polymer, is caused to increase beyond its inherent threshold limit, which thereby causes the electrical resistance to increase quite abruptly and dramatically, to very significantly reduce the flow of electrical current through the resistor, and virtually "trip" the circuit. While such tripping action does not in fact "open" the circuit to literally stop the flow of current as in the case of a normal fuse or circuit breaker, the increase in resistance is so abrupt, dramatic and significant that the flow of current is so significantly reduced that for all intents and purposes, the flow of current is virtually stopped. As a result, the circuit is protected from any possible damage as though the current flow had been completely interrupted, so that the PTC resistor does function much like a fuse or circuit breaker. When the temperature of the polymer is reduced back to the normal operating temperatures, the low resistance is again restored, and normal current flow is resumed. That is to say, once the fault has been corrected, or has corrected itself, so that the resistor's temperature will return to normal levels, the low resistance therethrough is restored, and accordingly, the PTC resistor automatically resets itself, so that operation of the circuit will immediately resume. The trip time can be as short as 0.1 second, and the electrical characteristics of the polymers, or PTC resistors, do not change significantly with repeated cycling through the critical temperature range; i.e., through the low resistance/high resistance transformations. This self-resetting feature has been shown to be a significant advantage in many applications, such as personal computers and peripheral equipment, telephones, battery packs, small motors in automobiles, and so on, in that in the event the circuit is tripped, there is no need to replace a fuse or reset a circuit breaker, and accordingly, no need for a costly service call if the equipment user is not sufficiently skilled to replace a fuse or reset a circuit breaker.
One disadvantage of state-of-the-art polymer PTC resistors, is that all are limited to operation at voltage levels of below about 60 volts. That is to say, none of the PTC resistors available commercially are capable of functioning as intended at voltage levels above about 50 to 60 volts. While differing PTC resistors are commercially available for differing types of applications, and classified to meet differing voltage and current requirements, none are commercially available for use in circuits where the driving voltage will, in normal operation, exceed about 60 volts. Accordingly, for those circuits normally expected to operate at voltage levels in excess of 60 volts, there are no polymer PTC resistors available that can provide over-current protection at such voltage levels, and the circuit designer must rely on conventional fuses or circuit breakers, and cannot therefore, take advantage of the automatic, self-resetting feature of PTC resistors.
U.S. Pat. No. 4,967,176, issued Oct. 30, 1990, and assigned on its face to Raychem Corporation, teaches and claims a sub-circuit assembly in which a plurality of PTC resistors are connected in series to provide a sub-circuit assembly which can be utilized at voltages higher than the voltage limits of the individual PTC resistors in the assembly. The patent acknowledges that it is known to connect a plurality of PTC resistors in parallel to achieve an overall current carrying capacity equal to the sum of the individual PTC resistors. The patent further acknowledges that while it is theoretically known to connect a plurality of PTC resistors in series to achieve an overall voltage limit equal to the sum of the voltage limits of the individual PTC resistors, that this result cannot in fact be achieved except for a very short period of time. Specifically, it is theorized that this result could be effected only if the resistors are exacting identical and all exposed to exactingly identical thermal environments, which is a condition virtually impossible to achieve and maintain. Indeed, even the smallest difference in electrical characteristics or thermal environments, under a fault condition, will certainly cause one of the PTC resistors to heat and thereby dramatically increase its resistance. Rather than tripping the circuit, however, the transformed PTC resistor will thereby shoulder nearly the entire voltage by itself and fail by melting or otherwise destroying the PTC resistor. Nevertheless, the patent teaches that such an increase in voltage capacity can be made to work for a rather short limited time, which may be useful in some applications where a slightly delayed response to an over-load condition to trip the circuit may be useful. The patent goes on to teach and claim a number of such applications where such a slight delay in tripping time can be used to an advantage. As an example, the patent teaches the use of such PTC resistors connected in series, used in combination with a conventional circuit breaker, so that the PTC resistors will carry the electrical current during the rather short interval of time while the circuit breaker contacts are moving apart as a result of a tripped circuit, to thereby avoid arcing or sparks before the contacts are fully separated. This, and none of the other disclosed applications, however, in any way contemplate the use of the PTC resistors connected in series solely for the purpose of protecting against over-current at higher voltage levels, as indeed, such protection could only be achieved for a very short period of time, as noted above. Accordingly, there remains a need for solid state, circuit protecting devices or current limiters that can function as do PTC resistors, but at driving voltage levels in excess of 50 to 60 volts.